To cast a metal object having an internal void, a mold defines the exterior surface and shape of the object, and a core defines the interior surface and shape of the internal void. These cores are most frequently made of sand mixed with a binder, which is set by thermosetting or chemical bonding into the desired shape of the core, as known in the art. The core is fixed in place in the mold, and molten metal is poured into the space between the core and the mold to form the object. The smoothness of the interior surface of the cast object resulting from this process is a function of the smoothness of the surface of the sand core. The surface of the sand core is made of grains of sand, which if untreated leave a fairly rough surface on the cast object. Moreover, the molten metal can penetrate and damage the untreated surface of the sand core, resulting in additional blemishes and undesirable features in the interior surface of the cast object. It is therefore desirable to fill the voids between the sand grains at the surface of the cores in order to produce a finished cast surface in those areas in contact with the sand core, better than would have resulted without the coating, and to use a material that will protect the sand core from the harsh environment of the casting process.
Various prior art processes have addressed this problem. In some processes, a dry refractory powder is mixed with an impacting media. This mixture is fluidized in an air-driven fluidized bed, typically in an enclosed chamber of some kind. The sand casting is immersed in the fluidized mixture. In other processes, vibratory drives are used to fluidize the mixture to permit complete immersion of the core in the mixture. The impacting media forces the refractory powder into the spaces between the grains of sand in the surface of the sand core. This provides a smoother surface of the core, which results in a smoother interior surface of the object. In addition, the refractory powder has a melt point exceeding that of the molten metal poured into the mold and thus protects the surface of the sand core from degradation caused by contact with the molten metal.
However, these techniques of coating with a dry powder have been limited to gravity casting, in which the mold is stationary and molten metal is poured into and simply fills the mold. The degree of attachment of the refractory material to the sand grains and the extent to which voids between the sand grains are filled is satisfactory for a gravity casting process, but has been found to be inadequate when the core is subjected to the particular forces encountered in the centrifugal process.
The general process of centrifugally casting a pipe will be described by way of example. The process of producing pipe by the centrifugal process utilizes a sand core to create detail in the enlarged end of the pipe casting to receive gaskets or seals used to seal the connection when one pipe is inserted into the next pipe in an installation. This sand core also serves as a restraint to prevent the molten metal from flowing out of the end of the mold during the casting process. The conditions to which such a sand core is subjected in this process are severe and distinctive from those in gravity casting processes. In the production of pipe by the centrifugal process, the sand core is inserted into the enlarged end of the metal pipe mold, either mechanically, or by hand, and then secured by a mechanical clamping device, or by manually inserting clamps or wedges of various configurations. The mold is then moved laterally around a trough designed to carry the molten metal from which the pipe is to be cast.
The mold then begins rotating at a defined speed and molten metal at a temperature in the range of about 2200 F (approximately 1200 C) to about 2800 F (approximately 1500 C) is introduced at the end of the trough opposite the cored end. The metal flows along the trough in a lateral direction and exits the trough in the vicinity of the sand core. The laterally flowing metal impacts the rotating core and flows against and along the surface of the core until the annular space between the core and the pipe mold is completely filled by the centrifugal force created by the rotating mold against the mass of the molten metal. The mold and the metal contained therein continue to rotate until the molten metal has sufficiently solidified to allow the pipe to be extracted from the mold. During the solidification process, the core is subjected to the centrifugal force of the molten metal.
The core is therefore subjected to two different forces of the molten metal. Initially, the metal washing against the surface of the core tends to erode the core, washing away sand grains at the surface of the core. Secondarily, the core is subjected to pressures created by the centrifugal force of the molten metal. This tends to force the molten metal into spaces between the sand grains at the surface of the core. During the solidification process, the binder utilized in the sand core is decomposed by the heat of the molten metal and the products of combustion migrate through the core to the annular interior of the core where they can easily exit the mold through its open end.
Known dry coating processes do not impart a coating of sufficient durability to withstand the forces to which a sand core is subjected during centrifugal casting. To adequately coat a core to be used in centrifugal casting with a suitable refractory, a wet coating process has been used. The refractory material is typically suspended in a fluid, water or various chemical solvents, and then applied by painting, spraying, dipping, or other such methods used for applying liquid coatings. Following the application of the liquid coating, the liquid portion must be driven off so as to leave a dry refractory surface which will be presented to the molten metal. To optimize the flowability, specific gravity, and other such characteristics of the refractory coating, as well as to assure the bonding of the coating to the core, and the filling of the voids between the cores, various minerals, clays, and other materials are added to the coating.
This wet coating process suffers from several problems. First, coating with a liquid refractory by spraying or brushing allows the possibility of incomplete coverage of the surface of the core, such as by “shadows” created by the complex surface contours of the core. Dipping of the core likewise allows the possibility of uneven coating thickness. Second, the coating must be dried before the core can be used. This adds a step to the manufacturing process. Additionally, the drying step requires the purchase, maintenance, and operation of additional capital equipment on the shop floor, adding to the cost of the finished product. The drying process itself introduces variables into the quality of the surface of the core, and thus of the casting. In particular, care must be taken during the drying process that the fluid used to create the coating does not penetrate the core, thus compromising the bond between sand grains, and also that the vapor created in the drying process does not separate the refractory from the surface of the sand grains. Significant measures for process control must be undertaken to address these variables and obtain acceptable results. The wet coating process often results in a cast surface which can have defects created by the lifting of the applied coating and causing it to be captured under the surface of the metal, and also penetration of the liquid metal though the coated surface, resulting in a rough surface of the cored portion of the pipe. Wet coated cores also leave a residue in the pipe, which further adheres to the metal, such as iron, during the annealing process and which must be cleaned out. Both of these conditions typically require subsequent grinding and gauging of the cored surface to meet specified dimensional and surface finish requirements, which adds process time and labor cost.
Thus, there exists a need for a process of centrifugally casting that utilizes a sand core having a coating applied by a dry coating process, which does not require a pre-casting drying step, and which yields a finished surface that does not require post-casting grinding and gauging. It also is desirous to coat the surface of the core with only the refractory material required to resist the forces applied to the core and to eliminate the use of liquids to apply the refractory, and other minerals, clays, and other materials which do not contribute to the resistive properties of the coating.